Multidrug resistant bacteria are becoming an increasing problem. Often antibiotic resistance genes are carried on conjugative plasmids i.e. plasmids that are capable of horizontal gene transfer between two bacterial cells in direct contact (=conjugation). Bacteriocins could become an alternative to antibiotics. The aim of this Master Thesis was to visualize the transfer of pOX38a plasmid from the donor to the recipient cell under the microscope, equipped with the optical tweezers. As we did not succeed, we next examined the possible reasons for this failure. We determined the frequency of the pOX38a conjugal transfer and influence of minimal medium and bacterial carriage on conjugation frequency. Our results showed that neither minimal medium nor the bacterial carriage affected the conjugation frequency. A further objective of the Thesis was to determine the effect of colicin E7 on the recipient strain. We were interested, if the recipient cell lysed after addition of colicin E7 to the bacterial culture and if the cell membrane permeability was changed in the way that propidium iodide can enter the cell. We found that a high enough concentration of colicin E7 has a bactericidal activity resulting in decrease of the optical density of the bacterial culture at a wavelength of 600 nm and lower number of CFU/mL. However cells did not lyse and the membrane permeability for propidium iodide was not changed, so propidium iodide did not enter the cells. After 24 hours of incubation with colicin E7 some cells acquired colicin E7 resistance, as optical density and number of CFU/mL increased again.